usb: xhci: tegra: Add runtime PM support

Add runtime PM support to the Tegra XHCI driver and move the function
calls to enable/disable the clocks, regulators and PHY into the runtime
PM callbacks.

Signed-off-by: Jon Hunter <jonathanh@nvidia.com>
Acked-by: Thierry Reding <treding@nvidia.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>